Snooze Button: Newsletter Challenge (08/02/05)

My guess is that the people who originally programmed digital clocks were to same ones who set us up for the Y2K scare by using only two year digits.

Good Morning!!!

The clock makers didn't want you to see how they do it, but the secret is that they stick a little flag on the back of the digit flap; not the one in front, but the one right behind it, so as the little wheel turns the little flag comes back up to the top in 9 minutes, and as it flops back down it flips the pawl that lets the weight fall a little... that lets the hammer ring the small bell.

Why the digital folks did the same thing, that's easy! They had an installed base!

--Paul

My guess is that it is to increase the effectiveness of the alarm itself. An alarm is only effective if it wakes you up and allowing you to sleep more than 10 minutes will bring you to REM (rapid eye movement) capable sleep, which is the most deep sleep, making is MORE difficult to wake you up. Regardless if the mechanism is cheap, no fancy alarm clock will be so good if it does not actually wake you up. This interval is just one trick to increase its success rate and a dirty one at that.

Correction: It's SWS (Slow Wave Sleep) not REM that is most likely the culprit at ~15 mins. In short, though, deep sleep can occur, making you less perceptive to the noise it makes and not as effective in its persuit to wake you.

Electric clocks must run synchronously to the power line frequency, or they'll be terrible inaccurate. Likewise, digital clocks may also use the power line to trigger a counter-timer circuit and increment the time at a determined number of cycles. I note that nine minutes = 540 seconds = 32,400 power line cycles at 60 Hz. This is just under 32768 which is 2 to the 15th power. If I needed to build a snooze circuit, I might be tempted to use a 16-bit counter that re-started the alarm when the 16th bit went high, just 32768 power line cycles after hitting the snooze button, or 9 minutes, 6.13 seconds later.

There's something strange going on here: nearly all alarm clocks in the UK (well that I've seen) have a 6 minute snooze time.

2 to the 14th is 16384, which, at 50 Hz is about 5 minutes and 46 seconds.

Same reason as above, diffent flag bit.

I make that 5.46 minutes, but a straw poll of one (the guy next to me) has the snooze time at 5 minutes so I guess the rationale holds.

All the digital alarm clocks I've owned over the last 25 years have had 9 minute snooze times...and all of them were bought in the UK. Even the £3.99 Tesco value one I bought last month has a 9 minutes snooze.

And if you use the snooze button to turn the radio on at other times of the day, it always lasts for 59 minutes - the same as the time the radio stays on when you use it as the alarm instead of the buzzer.

I always assumed that these were something to do with circuit design and not going up into the next set of units. 9 is the largest single digit integer and 59 the largest number of minutes less than an hour.

As written in the 8/09 issue of Specs & Techs from GlobalSpec:

By setting the snooze time to 9 minutes, the alarm clock only needs to watch the last digit of the time. So, if you hit snooze at 6.45, the alarm goes off again when the last digit equals 4 — fairly simple (and inexpensive) circuit logic. If the delay was 10 minutes, the alarm would either go off right away or it would take more circuitry to behave correctly. Another one of those ways in which you get what you pay for...